reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux.git/commit/?id=b88f2bfe111b8778c8bba9e0309c653966171bdd
        git remote add mcgrof https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux.git
        git fetch --no-tags mcgrof blk-iobuf-pool-v2
        git checkout b88f2bfe111b8778c8bba9e0309c653966171bdd
        # apt-get install llvm-dev

        # save the config file
        make W=1 ARCH=i386 olddefconfig
        make W=1 ARCH=i386 
        make W=1 headers_install
        make W=1 -C tools/testing/selftests